A wide variety of the physical properties of materials, such as ferroelectricity, ferromagnetism, piezoelectricity, conductivity, and dielectric permittivity depend upon material anisotropy and are therefore strongly affected by crystallographic texture, as reported in M. D. Vaudin, et al., “Accuracy and Reproducibility of X-ray Texture Measurements on Thin Films,” Mat. Res. Soc. Symp. Proc., Vol. 721, entitled “Magnetic and Electronic Films-Microstructure, Texture and Application to Data Storage,” edited by P. W. DeHaven et al. (Mat. Res. Soc., Warrendale Pa., 2002) pp. 17-22. With the appropriate choice of thin film texture, device operating efficiency and reliability can be strongly affected. Therefore, texture is a critical factor for thin film process control and is fundamental to device reproducibility
Microelectromechanical systems (MEMS) have been used in a wide range of applications, for example, from pressure sensors and accelerometers to microphones and digital displays. In 2006, STMicroelectronics and Nintendo revolutionized the entire MEMS industry through the launch of the Nintendo Wii gaming console that uses 3-D MEMS accelerometers for motion control. Since then, MEMS devices have been used in almost all new technology from smart phones to tablet PCs. In 2010, the MEMS industry experienced a 25% growth with the top four MEMS suppliers, Texas Instruments, Hewlett-Packard, Robert Bosch, and STMicroelectronics, increasing MEMS sales by 37%. Years of materials research has led to the current progress in MEMS technology. Various journal articles and books that have been written on piezoelectric thin films, one of the types of materials used in MEMS devices for micro-scale actuation
Lead zirconate titanate (Pb (Zrx Ti1-x)O3 or PZT) exhibits piezoelectric properties in thin films and is the most widely used piezoelectric bulk ceramic with ferroelectric properties. Examples of the use of thin films of PZT (used to create large force, large displacement actuators) include actuators for RF switches, relays and inkjet print heads.
The piezoelectric coefficient of PZT is inherently linked to its crystalline quality. The crystallographic texture of lead zirconate titanate (PZT) thin films strongly influences the piezoelectric properties used in MEMS applications. For tetragonal phase PZT films poled to saturation, the piezoelectric response is sequentially greater for random, {111}, and {001} texture. Textured growth can be achieved by relying on crystal growth habit and can also be initiated by the use of a seed layer that provides a heteroepitaxial template. Template choice and the process used to form it determine the structural quality and ultimately influence performance and reliability of PZT MEMS devices such as switches, filters, and actuators. {111}-textured PZT is generated by a combination of crystal habit and templating mechanisms that occur in the PZT/bottom-electrode stack.
The highest magnitude piezoelectric coefficients are observed at the PZT morphotropic phase boundary (MPB), where the crystal structure changes abruptly between the tetragonal and rhombohedral symmetry. At the MPB, the dielectric permittivity and piezoelectric coefficients reach a maximum. The MPB is located approximately at PbZr0.52Ti0.48O3, or PZT (52/48), composition. In thin film form, the composition and the crystalline texture must be controlled to achieve the maximum piezoelectric coefficients. For PZT (52/48), the highest coefficients are reported for a {001} textured PZT (52/48). The increased piezoelectric response and poling efficiency near to x=0.52 is due to the increased number of allowable domain states at the MPB. At this boundary, the 6 possible domain states from the tetragonal phase <100> and the 8 possible domain states from the rhombohedral phase <111> are equally favorable energetically, thereby allowing a maximum 14 possible domain states.
To date, two approaches have been taken to produce PZT thin film devices with the spontaneous polarization normal to the plane of the film and thus normal to the planar capacitor device, i.e. {001}-orientation. It is noted that directions in crystal lattices are defined in terms of directions l, m, and n, known as the Miller indices. Indices {l,m,n}, {100}, {010} and {001} represent planes orthogonal (normal) to the l, m, and n directions, respectively. The crystallographic directions are lines linking nodes (atoms, ions or molecules) of a crystal. Similarly, the crystallographic planes are planes linking nodes. Some directions and planes have a higher density of nodes; these dense planes have an influence on the behavior of the crystal. The notation {001} denotes the set of all planes that are equivalent to (001) (as shown in FIG. 1A) by the symmetry of the lattice. Heteroepitaxial growth makes use of a crystal substrate to initiate growth of an overlying crystalline material that has a different crystal structure than the substrate. Either a polar or a non-polar substrate may be used to initiate growth of a polar film. Further discussion of spontaneous polarization is found in FIG. 8 of U.S. Patent Application Publication No. 2010/0006780 and U.S. Pat. No. 7,956,369.
The first approach to producing PZT thin film devices with the spontaneous polarization normal to the plane of the film is to use a single crystal substrate and grow epitaxial layers of the bottom electrode and ferroelectric layer. The difficulty with this approach is that it places very strict requirements on the single crystal substrate, and the electrode must provide an epitaxial relationship with both the single crystal substrate and the ferroelectric film. The second approach to obtaining {001}-orientation is to use a seed layer and/or variations in process conditions to produce a {001}-textured PZT film whereby the PZT {001}-planes lie parallel to the substrate plane, but the relative orientations of the grains are randomly rotated about the substrate normal having no defined crystallographic role in the relationship between the substrate, capacitor electrodes and the PZT.
An alternative approach proven to obtain highly {001}-textured polycrystalline PZT films with enhanced piezoelectric coefficients is to use a PbTiO3 (PT) seed layer on the surface of a {111} textured Pt electrode. Unfortunately, such seed layers introduce composition gradients that cause a reduction in the reliability properties including polarization-retention and polarization-cycling endurance. When a seed layer such as PbTiO3 (PT) is used on the surface of the bottom electrode, annealing conditions, PZT texture, composition, and optimization of the piezoelectric properties are interdependent and cannot be altered independently.
In U.S. Pat. No. 6,682,772 to Fox entitled “High temperature deposition of Pt/TiOx for bottom electrodes,” hereby incorporated by reference, there is disclosed a platinum deposition method that uses a combination of an oxide adhesion layer and a high temperature thin film deposition process to produce platinum bottom electrodes for ferroelectric capacitors. TiOx was deposited on thermally grown SiO2 on a Si wafer. The platinum bottom electrode was deposited onto a TiOx layer at temperatures between about 300 and 800° C. Deposition at high temperatures changes the platinum stress from compressive to tensile, increases platinum grain size, and provides a more thermally stable substrate for subsequent PZT deposition.
Previous publications and patents do not provide a complete description of the {001}-textured PZT. It is insufficient to state independently just the percentage of {100}-textured grains or {001}-textured grains or angular distribution width of the textured grains. In order to fully define a texture and in the present case the crystallographic efficiency (or figure of merit) of the PZT film, the volume fraction of the {100}-oriented grains relative to the total volume of the PZT film must be defined. Of the {100} volume fraction, the volume fraction of the {001}-oriented grains must be defined. Finally, the misalignment distribution of the {001}-oriented grains relative to the substrate normal (which is also the electric field direction of the device) must be defined. When these crystallographic parameters are defined, a figure-of-merit for the material can be calculated.